The production of compounded plastics requires the use of low moisture content thixotropic agents to achieve satisfactory film integrity. For the purposes of the present invention the term "compounded plastic" is used to comprehend within its scope various forms and kinds of plastics which would include but is not limited to plastisols, adhesive sealants, coatings including compliance coatings and moisture cured coatings, and other organic binder systems which are heat set or heat cured or moisture cured.
Thixotropic agents are used in the production of compounded plastics which are applied to metal or other materials, for example, the coatings found on the wire baskets in a dishwasher or refrigerator. The use of the thixotrope permits the uniform application of the plastisol film to the object which is to be coated and the retention of that relatively uniform film during curing. In heat set (i.e., plastics which cannot be reformed) and heat cured (i.e., plastics which may be reformed by reheating) applications, the moisture content of the plastisols must be low enough to prevent the excessive release of moisture upon heating. Excessive amounts of moisture released during heating can cause bubbles to form in the plastisol coating, which causes reduced film continuity (a coating which contains voids), localized areas where the film does not adhere to the surface coated, and a less desirable appearance. Also, small quantities of moisture released during curing, although producing no visable blemish, can affect film integrity thereby causing, for example, reduced corrosion protection.
Changing requirements for coatings, including moisture cured (i.e., coating compositions that remain in the liquid state indefinitely when stored in a sealed container but are cured to the desired hardness when applied to the intended object, by moisture present in the atmosphere), heat set and heat cured primer and top-coat finishes, and higher solids compliance coatings to meet volatile organic compounds (VOC's) emissions standards present the coating formulations chemist with major constraints. The presence of free water in the pigments and other components limits the effectiveness of some recommended substitute compliance coatings. The problem of flow behavior of high solids coatings has not been completely solved. Additionally, poor humidity resistance of heat set or heat cured coatings or the premature activation of moisture cured coatings is the result of free water present in the pigments selected for the formulation.
Previously, the mineral asbestos processed by Johns-Manville Corporation, Union Carbide Corporation and others served as an effective thixotrope for plastisols, adhesive sealants, coatings and other organic binder systems. Thixotropic asbestos is processed to result in low moisture, typically less than 2% free moisture content. Asbestos had been considered a preferred cost effective thixotrope in these applications. Environmental and health considerations, as well as governmental regulations, limit its use and create a need for a suitable substitute thixotrope for use in heat set plastisol formulations and thermoplastic formulations, and compliance coatings.
Pyrogenic silica such as "CAB-0-SIL" by Cabot Corporation has been used as a substitute thixotrope for asbestos in plastisols, adhesive sealants, and other organic binder systems which are heat set or heat cured. The high cost of pyrogenic silica, however, creates an economic limitation on its use. Further, with time, pyrogenic silica is likely to absorb moisture, reducing its desirability as a thixotrope in, for example, plastisols.
Many clays, notably kaolinite clays, are used extensively as fillers in plastics. In such applications, the clays are used not as a thixotrope, but to achieve reduced shrinkage during curing, to obscure the fiber pattern of glass reinforcement and to increase resistance to chemical action and weathering.
Fuller's earth clays from the group consisting of sepiolite, palygorskite and attapulgite are effective thixotropes. Attapulgite, hydrated magnesium silicate clay processed by Engelhard Co., Floridin Co. and others is a particularly effective thixotrope. Thixotropic attapulgite and sepiolite are widely used as drilling mud in the drilling of oil wells. As commercially prepared, thixotropic attapulgite contains typically 10% to 15% moisture at 100.degree. C. Undesirable quantities of the moisture contained in typically prepared thixotropic Fuller's earth is released when used as a thixotropic agent in plastics such as plastisols and adhesive sealants, and other organic binder systems which are heat set, or moisture or heat cured. When typically prepared attapulgite is used in plastisols, for example, which are heat cured at about 100.degree. C., the moisture in the attapulgite volatilizes and deforms the structure of the plastisol, reducing its film continuity. This moisture release limits or precludes the use of typical attapulgite as a thixotropic agent for most heat set or heat cured applications.
Thixotropic grade attapulgite is processed commercially in carefully controlled drying ovens. The literature teaches that drying temperatures above approximately 400.degree. C. agglomerate the mineral particles and destroy the ability to disperse the clay in the form of colloidally active ultimate particles. The inventors do not know with certainty, but suspect that at this high temperature, refluxing of the water back into the clay structure is prevented, causing sintering of the attapulgite.
Bentonite clays treated with amine surfactants can become suitable thixotropic agents when the treated bentonite is mixed with an alcohol, water or other hydroxyl contributing agent by the user. However, bentonite so treated is not a suitable thixotrope for heat set or heat cured plastisols, or compliance coatings, because of the excessive moisture content. Release of the moisture from the treated bentonite clays in heat set or heat cured plastisols results in an undesirable film having a deformed surface, reduced continuity, voids and localized areas of nonadhesion.
Thus, the present invention produces an economical and environmentally safe alternate to the principal thixotropic agent previously used, asbestos.